PROCESS FOR THE PREPARATION OF GLYCEROL CARBONATE

Abstract

This invention relates to aprocess for the preparation of glycerolcarbonate from the reaction of glycerol and a dialkyl-carbonate, for example dimethyl carbonate, or a cyclic alkylene carbonate. More specifically, the invention relates to a process where the synthesis of glycerolcarbonate is conducted in the presence of a homogeneous transesterificationcatalyst and involves the partial reaction of a glycerol reactant stream and a dialkyl carbonateor cyclic alkylene carbonate reactant stream and an intermediate step of alcohol by-product separation before further reaction in order to improve glycerol conversion and glycerol carbonate selectivity and yield.

Claims

1-20. (canceled)

21. A process for preparing glycerol carbonate comprising the steps of: (i) in a first reaction zone in the presence of a homogeneous transesterification catalyst, contacting and partially reacting a glycerol reactant stream with at least one member of a group consisting of: a) a dialkyl carbonate reactant stream, comprising greater than 80 wt. % dialkyl carbonate, and b) a cyclic alkylene carbonate reactant stream, comprising greater than 80 wt. % cyclic alkylene carbonate; (ii) separating at least a portion of the alcohol by-product formed from the reaction in step (i) from the reaction mixture so as to obtain an alcohol-containing by-product stream; (iii) reacting at least a portion of the remaining reactants in a second reaction zone in the presence of the homogeneous transesterification catalyst; and (iv) obtaining a glycerol carbonate product stream.

22. A process according to claim 21, wherein the glycerol reactant stream is combined with the homogeneous transesterification catalyst prior to being fed to the first reaction zone.

23. A process according to claim 21, wherein the homogeneous transesterification catalyst is present in the reaction mixture in an amount from at least one member of a group consisting of: 0.25 to 5 wt % based on the mass of glycerol fed to the first reaction zone, and from 0.5 to 1.5 wt %, based on the mass of glycerol fed to the first reaction zone.

24. A process according to claim 21, wherein contacting and reacting in step (i) achieves at least one member of a group consisting of: from 50 to 90% glycerol conversion, from 70 to 90% glycerol conversion, from 80 to 90% glycerol conversion, greater than 50 wt. %, 75 wt. % of the alcohol by-product is removed in step (ii), and greater than 95 wt. % of the alcohol by-product is removed in step (ii).

25. A process according to claim 21, wherein the molar ratio of dialkyl carbonate and/or cyclic alkylene carbonate to glycerol fed to the first reaction zone is in the range of 1:1 to 3:1, 1:1 to 2.0:1, or in the range of 1.1:1 to 1.4:1.

26. A process according to claim 21, wherein: a) at least one of the first and second reaction zones are operated at a temperature from at least one member of a group consisting of: 40 to 160 C., 60 to 140 C., and 80 to 120 C.; b) the first reaction zone is operated at a pressure of at least one member of a group consisting of: from 10 kPa absolute to 1,500 kPa absolute (0.1 to 15 bar absolute), from 100 kPa absolute to 1,000 kPa absolute (1 to 10 bar absolute), and from 200 kPa absolute to 600 kPa (2 to 6 bar absolute); or c) the second reaction zone is operated at a pressure of at least one member of a group consisting of: from 5 kPa absolute to 150 kPa absolute (0.05 to 1.5 bar absolute), from 10 kPa absolute to 100 kPa absolute (0.1 to 1 bar absolute), and from 15 kPa absolute to 50 kPa absolute (0.15 to 0.5 bar absolute).

27. A process according to claim 21, further comprising a step of introducing at least one member of a group consisting of: further dialkyl carbonate for reaction in the second reaction zone to replenishing dialkyl carbonate lost during alcohol separation step (ii) and further cyclic alkylene carbonate for reaction in the second reaction zone to replenish cyclic alkylene carbonate lost during alcohol separation step (ii).

28. A process according to claim 27, wherein at least one member of a group consisting of: the molar ratio of dialkyl carbonate is higher in the second reaction zone than in the first reaction zone and acyclic alkylene carbonate to glycerol is higher in the second reaction zone than in the first reaction zone.

29. A process according to claim 21, wherein the process comprises continuous removal of alcohol by-product as it is formed in the second reaction zone.

30. A process according to claim 21, comprising at least one of: a) the dialkyl carbonate reactant stream comprises: i) greater than 90 wt. % dialkyl carbonate; ii) less than 5 wt. % alcohol; and iii) less than 2 wt. % water; and b) the cyclic alkylene carbonate reactant stream comprises: i) greater than 90 wt. % cyclic alkylene carbonate; ii) less than 5 wt. % alcohol; and iii) less than 2 wt. % water.

31. A process according to claim 21, wherein the process further comprises a step of recovering the homogeneous transesterification catalyst from the glycerol carbonate product stream using a cation exchange resin.

32. A process according to claim 21, wherein a stream comprising an azeotropic mixture of dialkyl carbonate reactant/cyclic alkylene carbonate and by-product alcohol is obtained as a result of the process; wherein the process further comprises a step of separating the unreacted dialkyl carbonate/cyclic alkylene carbonate from the azeotropic mixture to form a dialkyl carbonate/cyclic alkylene carbonate recycle steam; and wherein the dialkyl carbonate/cyclic alkylene carbonate recycle stream is used as a source of dialkyl carbonate/cyclic alkylene carbonate reactant for the process.

33. A process according to claim 21, further comprising at least one of: a) a dialkyl carbonate reactant stream is employed in the process, wherein the dialkyl carbonate reactant is selected from dimethyl carbonate, diethyl carbonate or mixtures thereof; and b) a cyclic alkylene carbonate reactant stream is employed in the process, wherein the cyclic alkylene carbonate is of Formula I below: wherein: ##STR00020## R.sub.1 is a divalent group, (CH.sub.2).sub.n, wherein n is an integer of from 2 to 6, and which is unsubstituted or substituted by at least one C.sub.1 to C.sub.6 alkyl group.

34. A process according to claim 21, wherein the homogeneous transesterification catalyst is selected from alkali metal carbonate, alkali metal bicarbonate, alkali metal hydroxide, alkali metal oxide, alkali metal alkoxide, alkali metal aluminate, alkali metal silicate alkaline earth metal carbonate, alkaline earth metal bicarbonate, alkaline earth metal hydroxide, alkaline earth metal oxide, alkaline earth metal alkoxide, alkaline earth metal aluminate, alkaline earth metal silicate and combinations thereof.

35. A process according to claim 34, wherein the homogeneous transesterification catalyst is selected from NaOMe, CaO, NaAlO.sub.2, Na.sub.2SiO.sub.3 or combinations thereof.

36. The process according to claim 21, wherein the homogeneous transesterification catalyst is a basic ionic liquid of the formula:
[Cat.sup.+][X.sup.] wherein: [Cat.sup.+] represents one or more cationic species; and [X.sup.] represents one or more basic anionic species; wherein: [Cat.sup.+] comprises: a) an acyclic cation selected from:
[N(R.sup.a)(R.sup.b)(R.sup.c)(R.sup.d)].sup.+, [P(R.sup.a)(R.sup.b)(R.sup.c)(R.sup.d)].sup.+, and [S(R.sup.a)(R.sup.b)(R.sup.c)].sup.+, wherein: R.sup.a, R.sup.b, R.sup.c, and R.sup.d .sub.are each independently selected from a C.sub.1 to C.sub.30, straight chain or branched alkyl group, a C.sub.3 to C.sub.8 cycloalkyl group, or a C.sub.6 to C.sub.10 aryl group; and wherein said alkyl, cycloalkyl or aryl groups are unsubstituted or may be substituted by one to three groups selected from: C.sub.1 to C.sub.6 alkoxy, C.sub.3 to C.sub.8 cycloalkyl, C.sub.6 to C.sub.10 aryl, C.sub.7 to C.sub.10 alkaryl, C.sub.7 to C.sub.10 aralkyl, CN, NO.sub.2, C(S)R.sup.x, CS.sub.2R.sup.x, SC(S)R.sup.x, S(O)(C.sub.1 to C.sub.6)alkyl, S(O)O(C.sub.1 to C.sub.6)alkyl, OS(O)(C.sub.1 to C.sub.6)alkyl, S(C.sub.1 to C.sub.6)alkyl, SS(C.sub.1 to C.sub.6alkyl), NR.sup.yR.sup.z, or a heterocyclic group, wherein R.sup.x, R.sup.y and R.sup.z are independently selected from hydrogen or C.sub.1 to C.sub.6 alkyl; or b) an aromatic heterocyclic cationic species selected from: benzimidazolium, benzofuranium, benzothiophenium, benzotriazolium, diazabicyclodecenium, diazabicyclononenium, diazabicyclo-undecenium, dithiazolium, imidazolium, indazolium, indolinium, indolium, oxazinium, oxazolium, iso-oxazolium, oxathiazolium, phthalazinium, pyrazinium, pyrazolium, pyridazinium, pyridinium, pyrimidinium, quinazolinium, quinolinium, iso-quinolinium, quinoxalinium, tetrazolium, thiadiazolium, iso-thiadiazolium, thiazinium, thiazolium, iso-thiazolium, triazinium, triazolium, and iso-triazolium; and wherein: [X.sup.] comprises an anion selected from alkyl carbonate, hydrogen carbonate, carbonate, hydroxide, alkoxide, chloride, bromide, nitrate and sulphate.

37. A process according to according to claim 36, further comprising at least one of: [Cat.sup.+] comprises a cation selected from: ##STR00021## and [X.sup.] comprises an anion selected from alkyl carbonate.

38. A process according to claim 37, wherein [X.sup.] comprises an anion selected from [MeCO.sub.3].sup..

39. A process according to claim 21, wherein the homogeneous transesterification catalyst is: a) an organic acyclic amine selected from tert-butylamine, isopropylamine, triethylamine, ditertbutylamine, diisopropylamine, diisopropylethylamine, dicyclohexylamine, dibenzylamine, benzyldimethylamine, diacetylchlorobenzylamine, dimethylphenethylamine, 1-dimethyl amino-2-phenylpropane and N,N,N-tritert-butylpropanediamine; or b) a substituted piperidine derivative having two to six C.sub.1-C.sub.4 alkyl substituents and where at least two of the alkyl substituents are located on carbon atom(s) adjacent the nitrogen atom of the ring.

40. A process according to claim 39, wherein the homogeneous transesterification catalyst is selected from 1,2,6-trimethylpiperidine, 2,2,6-trimethylpiperidine, 2,2,6,6-tetramethylpiperidine, 2,2,4,6-tetramethylpiperidine, 2,2,6,6-N-pentamethylpiperidine.

Description

[0148] The invention will now be described with reference to the following Examples and Figures wherein:

[0149] FIG. 1: is a schematic diagram illustrating a preferred embodiment of the process of the present invention employing two separate reactors;

[0150] FIG. 2: shows a plot of reactant and product composition and the effect of dimethyl carbonate ratio and reactant/product distribution;

[0151] FIG. 3: shows a plot of glycerol carbonate % in the final product obtained following by-product alcohol removal step (ii) and subsequent reaction in step (iii); and

[0152] FIGS. 4a to 4f:show plots of glycerol carbonate yield (%) for each of the samples taken from Streams A to F respectively against the number of equivalents of dimethyl carbonate present relative to unreacted glycerol at the start of each second stage reaction.

[0153] With reference to FIG. 1, a dimethyl carbonate (DMC) reactant stream (1) is fed to a mixing vessel (101). A glycerol (GLY) reactant stream (2) is also fed to mixing chamber (101). Preferably, as illustrated in FIG. 1, the glycerol reactant (GLY) stream (2) is dosed with transesterification catalyst (CAT), prior to combining with dimethyl carbonate in the mixing vessel (101). A mixed reactant stream (3) is withdrawn from mixing vessel (101) before being fed to the reaction zone of the first reactor (R1). The first reactor (R1) operates at above ambient temperature and the glycerol (GLY) and dimethyl carbonate (DMC) reactants are partially reacted therein. Typically, this stage of the process can achieve no more than 90% conversion of glycerol (GLY) to glycerol carbonate (GLC), but with near 100% selectivity for glycerol carbonate (GLC) product.

[0154] Following the incomplete or partial reaction in the first reactor (R1), a by-product methanol removal step is undertaken. A stream (4) comprising the reaction mixture is withdrawn from the first reactor (R1) and fed to column (102) for separation. Column (102) may, for example, be a distillation column or flash column. An overhead methanol (MeOH) rich stream (5) is withdrawn from column (102). Separated methanol (MeOH) rich stream (5) typically comprises an azeotropic mixture of methanol (MeOH) and dimethyl carbonate reactant (DMC). The azeotropic mixture may be separated in a subsequent step so as to provide a dimethyl carbonate (DMC) recycle stream.

[0155] The bottoms product of column (102), comprising the remaining components of the reaction mixture, is withdrawn as stream (6). If necessary, and depending on the dimethyl carbonate (DMC) content of the azeotropic mixture withdrawn from the top of column (102) as stream (5), additional dimethyl carbonate reactant (DMC) may be added by means of stream (7) to replenish dimethyl carbonate (DMC) lost in the methanol (MeOH) separation step. Stream (8) may therefore comprise a mixture of glycerol carbonate product (GLC), unreacted glycerol (GLY), homogeneous transesterification catalyst and a replenished amount of dimethyl carbonate (DMC).

[0156] Stream (8) is fed to the reaction zone of the second reactor (R2) where the second stage of reaction occurs. (R2) operates at above ambient temperature and unreacted glycerol (GLY) and dimethyl carbonate (DMC) reactants are reacted therein. Preferably, as illustrated in FIG. 1, reactor (R2) is a reactive distillation column having the capability for continuous by-product methanol (MeOH) removal, which is withdrawn from the column as stream (9). Methanol (MeOH), or a methanol (MeOH) and dimethyl carbonate (DMC) azeotrope, removed in stream (9) may be combined with stream (5) withdrawn from the column (102) as part of the methanol removal step and preferably fed to the same separation step for obtaining a dimethyl carbonate (DMC) recycle stream. Further reaction in reactor (2) advantageously provides further conversion of unreacted glycerol. By employing a reactive distillation column as reactor (R2) and ensuring continuous by-product methanol (MeOH) removal, selectivity for glycerol carbonate (GLC) is enhanced by modifying the dynamic chemical equilibrium to favour the formation of that product over by-products such as glycerol dicarbonate (GDC).

[0157] A glycerol carbonate (GLC) product mixture comprising glycerol carbonate (GLC), homogeneous transesterification catalyst, and any unreacted reactants and/or by-product methanol is withdrawn from reactor (2) as stream (10). As illustrated in FIG. 1, this stream is preferably fed to a transesterification catalyst removal unit (103), which is preferably in the form of a column packed with cation exchange resin. Stream (11) is withdrawn from the column, having had the transesterification catalyst removed from the mixture, before preferably being fed to column (104) where any unreacted dimethyl carbonate (DMC) and by-product methanol may be separated from the product mixture. Like column (102), column (104) may, for example, be a distillation column or flash column.

[0158] Unreacted dimethyl carbonate (DMC) and/or an azeotropic mixture of unreacted dimethyl carbonate (DMC) and any remaining by-product methanol (MeOH) are removed as stream (12). Any azeotropic mixture of methanol (MeOH) and dimethyl carbonate (DMC) which may be obtained in stream (12) may be combined with stream (5) withdrawn from the column (102) and/or stream (9) withdrawn from the second reactor (R2) as part of the methanol removal step and preferably fed to the same separation step for obtaining a dimethyl carbonate (DMC) recycle stream. As illustrated in FIG. 1, streams (5), (9) and (12) are fed to separation unit (105), which may, for instance, be an extractive distillation column or a column configured for pressure swing distillation.

[0159] Where the second reactor (R2) is a reactive distillation column, an additional step of separating unreacted dimethyl carbonate and/or methanol by-product from the glycerol carbonate product stream by means of column (104) is not typically necessary since it is expected that all unreacted dimethyl carbonate and methanol by-product will instead have been separated from the glycerol carbonate product during the reactive distillation. Thus, in that case, column (104) will be absent from the apparatus and stream (9) will instead comprise all of the residual dimethyl carbonate and by-product methanol.

[0160] Following separation of methanol (MeOH) and dimethyl carbonate (DMC) from the azeotrope in separation unit (105), a dimethyl carbonate (DMC) recycle stream (14) is withdrawn from the separation unit (105) as well as a methanol stream (15). Dimethyl carbonate (DMC) recycle stream (14) may be used to supply dimethyl carbonate (DMC) to stream (1) or mixing vessel (101). Additionally or alternatively, dimethyl carbonate recycle stream (14) may supply dimethyl carbonate (DMC) stream (7) which is used to replenish dimethyl carbonate (DMC) levels for the second stage reaction in the second reactor (R2). The bottoms product of column (104) is withdrawn as stream (13), corresponding to a purified glycerol carbonate (GLC) stream.

EXAMPLES

[0161] Stream Analysis

[0162] The various reactant and product streams of the examples below were analysed by HPLC analysis using a refractive index detector. The stationary phase used for the HPLC was an organic acids column (Phenomenex Rezex ROAOrganic Acids H.sup.+), the mobile phase was 7.5% acetonitrile, 0.5 mM aqueous H.sub.2SO.sub.4 and ethylene glycol was employed as an internal standard.

Example 1

First Reaction Stage Prior to Intermediate Alcohol Separation

[0163] Several reactions of dimethyl carbonate with glycerol were investigated employing 1 wt % of NaOMe homogeneous transesterification catalyst and a reaction time of 1 hour at 80 C. Different ratios of dimethyl carbonate to glycerol (from 0.5:1 to 5:1) were employed in different reactions. Following HPLC analysis of the reaction mixture composition after this reaction time, the results were used to prepare a graph (FIG. 2) showing the effect of the number of equivalents of dimethyl carbonate (DMC) to glycerol reacted.

[0164] FIG. 2 corresponds to a graphical representation showing the relative proportions of glycerol carbonate (GLC), glycerol dicarbonate (GDC) and glycerol (GLY) against the number of equivalents of dimethyl carbonate reacted. The results demonstrate that lower dimethyl carbonate (DMC) to glycerol (GLY) ratios lead to less unreacted dimethyl carbonate (DMC) at the end of the reaction for optional recycle. The results also show that as the dimethyl carbonate to glycerol ratio is increased, the proportion of glycerol dicarbonate (GDC) produced also increases. Up to a point, increasing the dimethyl carbonate to glycerol ratio increases the amount of glycerol carbonate obtained within the reaction time, up to a maximum amount of approximately 90 mol %, based on the mass of glycerol (GLY), glycerol carbonate (GLC) and glycerol dicarbonate (GDC). However, beyond that point further increases in dimethyl carbonate to glycerol ratio leads to a preference for undesired glycerol dicarbonate formation.

Example 2

First Reaction Stage Followed by Intermediate Alcohol Separation Step

[0165] In a first reaction stage, a 1:1 molar ratio of dimethyl carbonate to glycerol were reacted, with 1 wt. % NaOMe homogeneous transesterification catalyst based on the amount of glycerol fed to the reactor being dissolved in a glycerol reactant stream prior to reaction. The reactor was operated at 80 C. for 1 hour before the reaction mixture was withdrawn and fed to a distillation column for methanol separation. Several methanol separations were completed for different samples taken from the first reaction stage using a distillation column operating at a temperature of 40 C. but at different pressures (ranging from 40 to 120 mbar). Yield of glycerol carbonate following the methanol separation was determined by HPLC.

[0166] FIG. 3 corresponds to a graphical representation showing glycerol carbonate yield (%) (which also corresponds to the molar proportion relative to glycerol in this case as 100% selectivity was observed) for each sample against the operating pressure of the distillation column. The results demonstrate that, across the range of pressure investigated, increasing the pressure in the distillation column led to greater yield of glycerol carbonate (GLC) prior to further reaction in a second reactor. This trend may be observed up until the maximum practical pressure is implemented for the particular distillation temperature of the distillation column. Thus, where a higher distillation temperature is implemented, a similar trend in glycerol carbonate yield would be observed, but for a range of commensurately higher distillation pressures.

Example 3

Second Stage Reaction (without Continuous By-Product Alcohol Removal)

[0167] Various reactant stream compositions representative of partially reacted streams were tested to determine the effect of the second reaction stage. Each of the streams included a particular molar ratio of glycerol carbonate (GLC) to unreacted glycerol (GLY), as indicated in Table 1 below, and 1 wt. % of a homogeneous transesterification catalysts, based on the amount of unreacted glycerol prior to the second stage reaction.

TABLE-US-00001 TABLE 1 Molar ratio of Transesterfication Stream GLC:GLY catalyst A 90:10 NaOMe B 80:20 NaOMe C 70:30 NaOMe D 90:10 TMDH-piperidine E 80:20 TMDH-piperidine F 70:30 TMDH-piperidine

[0168] Samples of each of the streams A to F were reacted at 100 C. in a round bottomed flask reactor with various equivalents of dimethyl carbonate, relative to the amount of unreacted glycerol initially present in the sample, for 2 hours in each case to ensure the equilibrium point was reached.

[0169] FIGS. 4a to 4f correspond to graphical representations showing glycerol carbonate yield (%) for each of the samples taken from streams A to F respectively against the number of equivalents of dimethyl carbonate present relative to unreacted glycerol at the start of each reaction. FIGS. 4a to 4f also show the relative proportion of glycerol (GLY) and glycerol dicarbonate (GDC) present following the reaction for each sample. FIGS. 4a to 4f consistently show that by increasing the number of equivalents of dimethyl carbonate present, glycerol conversion to product is generally increased. However, as the number of equivalents of dimethyl carbonate is increased, selectivity for unwanted glycerol dicarbonate by-product also increases.

Example 4

Second Stage Reaction (with Continuous By-Product Alcohol Removal)

[0170] A sample from the second stage reaction of Stream A experiment from Example 3 which had been reacted for 2 hours at 100 C. with 1.5 equivalents of dimethyl carbonate was taken and its product distribution analysed before it was subjected to further reaction by heating together with continuous by-product methanol removal (an open reactor vessel being used allowing volatile components to evaporate) Following the further reaction, the product distribution was also analysed at that stage. Results from the analysis conducted in respect of the sample taken from the second stage, before and after further reaction with continuous methanol removal are provided in Table 2 below. Results correspond to the relative proportions of glycerol, glycerol carbonate and glycerol dicarbonate present in the mixture tested.

TABLE-US-00002 TABLE 2 Sample from Product Stream of Product Stream of Reactor Stage 2 Reactor Stage 2 further reacted Product Stream of (without continuous with continuous Component Reactor Stage 1 alcohol removal) alcohol removal Glycerol 90.0% 93.5% 97.5% Carbonate Glycerol 10.0% 4.0% 2.5% Glycerol 2.5% Dicarbonate

[0171] The results of Table 2 demonstrate that the composition of the feed to the second stage reactor, comprising a 90:10 ratio of glycerol carbonate to unreacted glycerol (and no glycerol dicarbonate), was changed as a result of reaction in the second stage. In particular, further glycerol conversion to product was observed. However, as can be seen from the results for the product stream of Reactor Stage 2, selectivity for glycerol carbonate for this stage of the reaction was comparable to the selectivity for glycerol dicarbonate. However, further reaction of that sample in a reactor configured for continuous by-product methanol significantly modifies the product distribution. In addition to further glycerol conversion, the further reaction of the second stage sample effectively converts glycerol dicarbonate by-product present in the composition to the desired glycerol carbonate product. A comparable product distribution is also obtained if Reactor Stage 2 is instead operated with continuous alcohol removal initially. This experiment demonstrates that incorporating continuous by-product methanol removal into the second stage of the reaction surprisingly enhances both glycerol conversion as well as selectivity for glycerol carbonate formation over glycerol dicarbonate.